It has hundreds of crossing points and the removal of the fortified border checkpoints which used to monitor north south traffic is seen as one of the lasting achievements of the peace process.

Re-creating "unapproved crossings" or putting those physical posts back to police a new customs regime would be a security nightmare, providing a focal point for dissident republicans intent on reviving the "Troubles".

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The operation of the Irish border is one of the most sensitive Brexit issues

For months, it's been the joint mantra from both Dublin and London - that after Brexit, the border between Northern Ireland and the Republic of Ireland should be as "seamless and frictionless as possible".

That phrase hinted both at an ideal flexible arrangement in the future and an ideal shared approach from both the UK and Irish governments, together stressing to the EU the paramount importance of doing nothing that might constitute any kind of risk to the political progress made over recent decades.

So the cheque is in the post after the DUP agreed to back Theresa May's minority government in Commons votes.

As a result, Northern Ireland will receive an extra £1bn during the next two years as part of the deal, but what could prevent the Stormont parties setting up a power-sharing executive to spend the money?

RHI inquiry

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The Renewable Heat Incentive scheme is approximately £490m over budget

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Northern Ireland voters last went to the polls in March

It has been a strange election campaign in Northern Ireland, coming so quickly after the March assembly contest and interrupting the talks that were meant to restore devolution.

The debate has ranged across the implications of Brexit for cross-border trade, the consequences of the political vacuum at Stormont and, rather less predictably, whether it is OK for one party leader to call another a "blonde".

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Arlene Foster originally made her "crocodile" quip while vowing there would be no Irish Language Act

If labelling Sinn Féin as "crocodiles" was the Arlene Foster phrase that stuck at the start of the spring assembly election campaign, then the DUP leader was determined not to make the same mistake again at the outset of the Westminster battle.

The ad lib "crocodile" comment emerged during answers to the press, so the DUP's Gavin Robinson was only half joking when he advised party activists that the longer they cheered the better, as it would cut down the time for those pesky enquiries from reporters.

About Mark

BBC journalist since 1980s. Reporter for Spotlight, Ireland Correspondent covering IRA ceasefire and Good Friday Agreement, United Nations Correspondent in New York, Stormont Political Editor since 2001.

Covered stories in Somalia, Sierra Leone, Tanzania and Israel.

Author of Flash Frames -12 Years Reporting Belfast and co -author of Man of War, Man of Peace: a biography of Gerry Adams

Once worked as a trainee reporter for Indian newspaper "The Hindu".

Educated in Oxford before going to university in Cambridge to study history